Hiya. the body's preferred fuel is glucose from carbs and the liver stores some that'd last us 2-3 days if our main source of fuel changed from carbs to our own fat stores as LT is designed to do. Some people go into ketosis quicker after maybe 24 hours and others take a bit longer, say 4-6 days. that's why they don't let you have flapjacks for 2 wks to make sure ketosis is well established. hth
It depends. There's 5 stages to ketosis. Ketosis itself happens at stage 4 (usually within 3 to 4 days after the last carby meal) and continues the shift well into the 2nd week. It takes approx 14 days for it all to shift.
But when glycogen stores are used up, there is a stage where the body uses glucose from other fuels available...then half of that and half ketones...then onto ketosis, then a reduction in protein breakdown (into week 2 there)
If you've been on a low carb you'd be in ketosis anyway, so wouldn't have to wait 3 to 4 days. If you were lowish carbs, or do a load of exercise before starting LT then you are already into the phases, so ketosis happens quicker as you would already be during or past stage 1/2
Not sure if that answers your question or not. D'oh.